Wireless energy transfer or wireless power is the transmission of electrical energy from a power source to an electrical load without interconnecting wires. Wireless transmission is useful in cases where interconnecting wires are inconvenient, hazardous, or impossible. Wireless power differs from wireless telecommunications, where the signal-to-noise ratio (SNR) or the percentage of energy received becomes critical only if it is too low for the signal to be adequately recovered. With wireless power transmission, efficiency is the more important parameter.
Two common forms of coupling in wireless power transmission are inductive coupling and resonant inductive coupling. A wireless power transfer system usually consists of electromagnetically coupled transmitting and receiving coils. Due to coil coupling, energy from the primary side can be transferred to the secondary side over a distance. Electromagnetic induction wireless transmission techniques are near field over distances comparable to a few times the diameter of the device or devices approaching one quarter of the wavelength used. Near field energy itself is non-radiative but some radiative losses do occur. In addition there are usually resistive losses. Energy transfer by induction is usually magnetic but capacitive coupling may also be achieved.
Electromagnetic induction works on the principle of a primary coil generating a predominantly magnetic field and a secondary coil being within that field so that a current is induced in the secondary. Coupling should be tight in order to achieve high efficiency. As the distance from the primary is increased, more and more of the magnetic field misses the secondary. Even over a relatively short range the induction method is rather inefficient, wasting much of the transmitted energy.
The action of an electrical transformer is the simplest instance of wireless power transmission by induction. The primary and secondary circuits of a transformer are not directly connected. Energy transfer takes place by electromagnetic coupling through a process known as mutual induction. Principal functions are stepping the primary voltage either up or down and electrical isolation. Mobile phone and electric toothbrush battery chargers, and electrical power distribution transformers are examples of how this principle is used. Induction cookers use this method. The main drawback to this basic form of wireless transmission is short range. The receiver must be directly adjacent to the transmitter or induction unit in order to efficiently couple with it.
Common uses of resonance-enhanced electrodynamic induction are charging the batteries of portable devices such as laptop computers, cell phones, medical implants, and electric vehicles. Resonance is used in both the wireless charging pad (the transmitter circuit) and the receiver module (embedded in the load) to maximize energy transfer efficiency. This approach is suitable for universal wireless charging pads for portable electronics such as mobile phones. It has been adopted as part of the Qi wireless charging standard. It is also used for powering devices having no batteries, such as RFID patches and contactless smartcards, and to couple electrical energy from the primary inductor to the helical resonator of Tesla coil wireless power transmitters.
Qi is an example of a system for inductive charging that uses the protocol established by the Wireless Power Consortium (WPC). Qi establishes a common language for inductive chargers and devices to talk to one another. So any device with a Qi-enabled accessory or with Qi built directly into it can charge on any Qi inductive charging pad.
Inductive charging is what happens when two devices—one designed to send power and the other designed to receive it—touch one another and energy is transferred between them. In the past, these two devices had to be designed specifically for each other; but devices and chargers designed to support the standard established by the WPC can be freely interchanged. The WPC standard allows the universal charging of compliant smartphones, cameras, mp3 players and anything else that uses up to 5 W without directly plugging in those devices. By using an electromagnetic field to transfer energy, charging pads are able to intelligently communicate back and forth with the devices they're charging.